The Bacillus cereus sensu lato group consists of spore forming bacteria that survive pasteurisation, and are associated with foodborne illnesses. This study enumerated presumptive B. cereus s. l. isolated from Tasmanian dairy environments; assessed their prevalence in ricotta and whole pasteurised milk; and phenotypically characterised dairy and ricotta sample isolates, including virulence markers.
Soil, faeces, milking cups, teats, and bulk raw milk were sampled from six Tasmanian dairies. Presumptive B. cereus s. l. was present in all soil (M = 4.5 ± 0.25log10CFU/g), all faecal (M = 3.2 ± 0.11log10CFU/g), 10/12 teat (M = 2.5 ± 0.39log10CFU/teat), and 8/12 milking cup (M = 0.50 ± 0.23log10CFU/milking cup) samples. Six negative bulk raw milk samples from three farms were enriched for 48hrs at 20˚C resulting in 4/6 samples positive for B. cereus s. l..
Five samples of ten brands (n=50) of 1L whole pasteurised milk, and five samples of four brands of ricotta (n=20) were analysed for B. cereus s. l. before and after incubation at 25˚C for 48hrs (20˚C for ricotta). Enterotoxin (NHE and HBL) production in milk was also assessed following incubation. Initially 22% of milk samples but no ricotta samples were B. cereus s. l. positive, increasing to 88% and 75% respectively following incubation. 88% and 66% of milk samples harboured isolates capable of producing NHE or HBL, respectively.
Phenotypic characterisation of 51 dairy farm isolates identified 8% B. thuringiensis, 14% B. mycoides, 37% B. cereus sensu stricto, and 41% B. weihenstephanensis. Among nine ricotta isolates, only B. cereus s. s. was identified. NHE was produced in sterile milk by 98% of dairy B. cereus s. l. isolates and 100% of ricotta isolates when incubated at 12˚C for 72hrs; and HBL in 3.9% of dairy isolates, and 0% of ricotta B. cereus s. l. isolates.
B. cereus s. l. are widely distributed throughout Tasmanian dairy farm environments, and isolates capable of NHE and HBL production are present at low concentrations in bulk raw milk, ricotta, and whole pasteurised milk. However, a high proportion of 1L whole pasteurised milk and 375g ricotta samples contain entero-toxigenic isolates.